Biology, religion and love came to the fore when the controversial issue of same-sex marriage was debated in front of MPs in Christchurch.

The city's Air Force Museum yesterday hosted the parliamentary select committee considering public submissions on the Marriage (Definition of Marriage) Amendment Bill, which would legalise same-sex marriage.

The committee's MPs heard from about 20 supporters and opponents of the bill, with many in favour of same-sex marriage drawing on their experiences of discrimination.

Otago University Students' Association queer support officer Neill Ballantyne said his own experience as a bisexual man amplified the hypocrisy of the current law.

"It seems absolutely ridiculous to me that if I fall in love and want to spend the rest of my life with a woman, I can [get married] but, if I fall in love and want to spend the rest of my life with a man, I only get the consolation prize of a civil union."

Sara Epperson reminisced about dressing up as a 4-year-old bride at Halloween, saying the gap between her childhood dreams and her recent civil union had to be bridged for the sake of gay New Zealanders.

"I had a tiny dress, a tiny bouquet and I was happy as I could be. Nobody thought to tell me that I might not be a real bride, and that my dreams might be compromised because of who I would grow up to be.

"Instead, I got civilly united - how romantic."

Same-sex marriage opponents spoke passionately against the law, many focusing on the religious origins of the institution.

David Black serenaded MPs with a waiata, before warning them that children would be the victims of a change to marriage.

"A child can't reason until they're 7, so how are they supposed to understand? What do they do on Mother's Day when they have two fathers? What do they do on Father's Day when they have two mothers?